Vapor discharge cell

ABSTRACT

An electrical discharge cell having a plural wick heat pipe for preventing contamination without depleting the supply of vapor source material.

111mm Mates Pmem 1151 3,654,567 Hodgson 51 Apr. 4, 1972 [541 VAPORDISCHARGE CELL 3,457,436 7/1969 Levedahl ..l65/105 x 3 460 524 8/1969 7l t 2] nven or Rodney T Hodgson, Somers, N Y 3,464,025 8/1969 [73]Assignee: International Business Machines Corpora- 3,484,720 12/1969tlon, Armonk, NY. 3,543,841 l2/197O 221 Filed: Dec. 31, 1970 3156313092/1971 3,576,500 4/1971 PP 1 3,240 3,585,524 6/1971 Silfvast ..33l/94.5

52 vs. C! ..331/94.5, 165/105, 313/180 'f Examiner-Mm wibe" 51 1111. C1..H01s 3/02 4mm"! Examiner-Edward Bauer 581 Field 01 Search .331/945;313/130; 165/105 Attorney-"1mm" and Jami" and Karl @558 [56] ReferencesCited [57] ABSTRACT UNITED STATES PATENTS An electrical discharge cellhaving a plural wick heat pipe for preventing contamination withoutdepleting the supply of Burggraaf et a1 ..33 vapor source material3,405,299 /1968 Hall et al ..165/ X 3,441,752 4/1969 Grover et al. /105UX 7 Claims, 2 Drawing Figures 11 FROM RESERVOIR TEMPERATURE CONTROL 0CE 8 UR RESERVOIR PATENTEDAPR 41972 3,654,567

m FROM RESERVOIR TEMPERATURE H2 CONTROL SOURCE T0 RESERVOIR 347 245L--FR0M RESERVOIR PRESSURE TEMPERATURE 244 E FEESJ CONTROL CONTROL n r23L 255 mfm T0 POWER EWESERVOIR SOURCE I 2"; 2 20? INVENTOR RODNEY T.HODGSON BY Mai/Luz AGENT vAPoa DISCHARGE CELL BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to electricdischarge cells in general and more specifically to vapor dischargecells suitable for use in a laser.

2. Description of the Prior art Vapor lasers are well-known in the priorart, however, they have prior to this invention, suffered from a problemof window contamination by condensing vapor. A prior art solution tothis problem was to allow the vapor to condense closer to theevaporization source and prevent vapor diffusion to the window with afew torr of pressure of neutral gas. An example of this method ofsolution is shown in FIG. 3, on page 34, of an article in the Aug., 1969issue of Laser Focus. A disadvantage of this method of reducing windowcontamination is that it depletes the reservoir of vapor material at thevaporization source, therefore, the life time of the device is limited.Another disadvantage is that the vapor pressure can vary from theoptimum value.

SUMMARY OF THE INVENTION It is an object of this invention to provide avapor boundary in a conducting vapor electrical discharge cell toprevent contamination in an improved manner.

It is a further object of this invention to prevent window contaminationin vapor lasers without depleting the vapor source material at thevaporization source.

It is a still further object of this invention to prevent windowcontamination in electrical discharge metal vapors lasers withoutproviding a parasitic current path which would short out the electricaldischarge power source.

I accomplished the above objects by employing an improved heat pipewithin the discharge chamber. When a conductive vapor source material ora conductive wick material must be used, I separate the wick into twoseparate wicks, each located near a window. Each provides both a vaporevaporization source and a vapor condensation sink, so that avapor-liquid circulation effect is set up near each window. My inventionincludes the discovery that any neutral gas which originally filled thedischarge chamber will be pumped out from the central area of thedischarge chamber between the wicks by the circulation flow of thevapor-liquid at each of the wicks.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an embodiment of myinvention. FIG. 2 shows an alternate embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS:

FIG. 1 shows how the wicks of a metal vapor laser are located within adischarge chamber.

Referring now to FIG. 1, discharge chamber 101 is shown as a tubularchamber. The tubular shape of chamber 101 is not critical to myinvention and an annular or other shape could also be used. Dischargechamber 101 is made of an insulating material which can withstand thehigh temperatures normally encountered within the chamber. Boronnitride, quartz and alumina are examples of high temperature insulatingmaterials suitable for constructing of discharge chamber 101.

A pair of wicks, 103 and 105 are located at each end of the dischargechamber 101 and are separated from each other by a predetermineddistance called a discharge path. When an electric discharge is set upwithin the discharge chamber, it will flow along this separatingdischarge path between wicks 103 and 105. Wicks 103 and 105 may be madefrom any number of materials having capillary properties and which arealso capable of withstanding the temperatures expected within thechamber. Examples of wick materials are nickel, stainless steel andtantalum. It is preferable in this embodiment, that the wick material beelectrically conducting, however, it is not absolutely necessary if thevapor source material which is absorbed within the wicks is anelectrical conductor.

Power source 107 is connected between wicks 103 and 105, so that it iselectrically connected in series therewith. In those situations, wherethe wick material is non-conducting, the electrical connection are madebetween the electrically conducting vapor source material and powersource 107 by extending the connection into the wick allowing it to beimmersed in the vapor source material. Power source 107 has AC and/or DCand/or pulsed current and voltage capabilities and is controllable forinitiating and maintaining an electrical discharge within the dischargechamber.

Alternatively, power can be coupled into the discharge chcnber byinduction heating using a radio frequency coil wrapped around the tube101.

A heat exchanger means 109 surrounds the discharge chamber for removingor adding heat to the discharge chamber 101 under control of temperaturecontrolling means 111 to which it is connected. Heat exchanger means 109may be any of a number of well-known exchanger means, such as acirculating liquid, an electrical heater, or a controllable heatsink/source such as a peltier pile. Temperature control 111 includes atemperature sensing element and a temperature controlling means 112connected to heat exchanger means 109 to control the temperature withindischarge chamber 101. The temperature is controlled to be at thecondensation temperature of the vapor source material at the end of eachwick near the windows. The electric discharge will raise the temperaturein the interior of the discharge chamber to a temperature which willprevent condensation of the vapor within the central portion of thedischarge chamber at the operating vapor pressure.

Plates 113 and 115 enclose the volume within discharge chamber 101, ifnecessary or desired. Plates 113 and 115 may be windows in someinstances and mirrors in others or merely free space such as a pressuresensing orifice. In either instance, the proximity of plates 113 and 115to wicks 103 and 105 prevents vapor from within discharge chamber 101from condensing on the inner surface of plates 113 and 115 or fromentering the free space to be protected.

OPERATION When in operation, the vapor source material will be a liquidwhich will be absorbed within wicks 103 and 105. Electrical energy isapplied by turning on power source 107 causing an electrical dischargebetween wicks 103 and 105. This electrical discharge heats the areabetween wicks 103 and 105 causing some of the vapor source material toevaporate. Because the temperature is highest toward the center of thedischarge chamber, material will evaporate from that region of wicks 103and 105 which is nearest to the center of the discharge chamber. Thatregion is called the evaporation region. After evaporating from theevaporation region of the wicks, the vapor will circulate as shown byarrows 117 and will condense on the outer region of wicks 103 and 105due to the cooler temperature in these regions of the discharge chamber101. As a result of the circulation process just described, atoms ofneutral gas existing within the central portion of chamber 101 willgradually pump out toward plates 115 and will be replaced by vapor.Temperature control 111 and heat exchanger 109 will operate to controlthe temperature within the discharge chamber 101 to be at thecondensation temperature nearest the windows. The heat pipe effect ofthe discharge chamber will keep the temperature in the central portionof chamber 101 near the wicks from becoming excessive. In applicationswhere the electric discharge need not be closely controlled forpurposes, such as to control a lasing action, the electric discharge canbe used to control the temperature within the chamber 101 and heatexchanger 109, then need only be a finned heat sink. Temperature control111 would then be connected to heat exchanger 109 to sense thetemperature but would also be connected to power source 107 in order tocontrol the energy being released within discharge chamber 101. In otherapplications, which require close control of the electric discharge,heat exchanger 109 will act both as a heat sink and/or as a heat sourceunder control of temperature control 111 to maintain the temperaturewithin the central portion of discharge chamber 101 at a temperatureabove the vaporization temperature of the source material, and thetemperature of the portions near plates 1 13 and 115 at or below thecondensation temperature of the source material. The thermal gradientfrom the central portion of the chamber 101 to plates 113 and 115 iscaused by the release of heat by the electric discharge, primarily, inthe central portion of chamber 101.

DESCRIPTION OF AN ALTERNATE EMBODIMENT The following description willset forth an alternate embodiment of the invention which is specificallyintended for use as the discharge chamber in a vapor laser. FIG. 2 showsa cross sectional view of the alternate embodiment.

Referring now to FIG. 2, discharge chamber 201 is shown as a tubularchamber, however, it is well-known that lasers need not be of a tubularshape and therefore, this invention is not to be taken as limitedthereto. Discharge chamber 201 is made from a strong insulating materialsuch as boron nitride, quartz, or alumina.

A pair of wicks, 203 and 205 are located at each end of the dischargechamber and separated from a material having capillary properties andwhich is also capable of withstanding the temperature expected withinthe chamber. Examples of wick material are nickel, stainless steel andtantalum.

A pair of guard rings, 233 and 234 are provided. Guard ring 233 islocated within wick 203 and guard 235 is located within wick 205 for thepurpose of producing a fairly high length to diameter ratio in theregion where the vapor flows.

A pair of electrodes 237 and 239 are provided in order to give a widerdesign latitude in choosing wick material and vapor source materialwhich may now be non-conducting. Electrode 237 is placed between wick203 and window 213. Electrode 239 is placed between wick 205 and 215.Electrodes 237 and 239, therefore, lie between and adjacent to windows213 and 215 respectfully and wicks 203 and 205 lie between theelectrodes. The distance between electrodes 237 and 239 define thedischarge path in this embodiment. Windows 213 and 215 are set atBrewsters angle in order to minimize reflection losses of light in thelaser optical cavity.

Power source 207 is connected between electrodes 237 and 239 so that itis electrically connected in series therewith. Power source 207 can beidentical to power source 107 and FIG. 1 and is also controllable forinitializing and maintaining an electric discharge within the dischargechamber which will cause the lasing action of the vapor to be vaporizedwithin chamber 201.

A heat exchanger means 209 surrounds each end of discharge chamber 201enclosing the portion of chamber 201 which contains the condensationarea of each wick. Heat exchanger means 209 is a heat sink whichmaintains the temperature of the area enclosed within it at or below thecondensation temperature of the vapor source material being used. Heatexchanger 209 will usually be a simple convection, conduction, and/orradiation type heat sink which is a mere matter of design choice.

A second heat exchanger 241 surrounds the central portion of dischargechamber 201 enclosing the volume of the chamber between the wicks aswell as that volume containing the evaporation regions of each wick.Heat exchanger 241 is connected to temperature control 243 for adding orremoving heat from the central portion of discharge chamber 201. Heatexchanger 241 may be a circulating liquid type or it may be acombination electrical heater and cooler, such as a peltier pile or anyother well-known exchanger capable of adding or removing heat fromchamber 201. Heat exchanger 241 is connected to and controlled by atemperature control 243 which detects the temperature within the centralportion of chamber 201 at sensing element 242 and maintains thistemperature at or above the temperature necessary to give the requiredvapor pressure of the source material being used by actuating transducermeans 244.

A gas inlet and exit orifice 205 is connected to the cavity withinchamber 201 and communicates with pressure control 247. Pressure control247 introduces and controls the pressure of a neutral gas within chamber201 thereby indirectly controlling the vapor density within chamber 201.A number of neutral gases are well-known to be suitable for use in avapor laser. Examples are helium, argon and neon.

OPERATION When the alternate embodiment of the invention is in operationas a laser, vapor source material to be vaporized is absorbed withinwicks 203 and 205. Electrical energy is applied by turning on powersource 207 and causing an electrical discharge between electrodes 237and 239. If necessary, temperature control 241 will cause heat exchanger241 to apply heat to the central portion of chamber 201, in order tobring the temperature therein to the vaporization temperature of thesource material. Many suitable lasing source materials are known.Examples are copper, and thallium. It is, of course, well-known that anelectrical discharge will create heat and this heat may be sufficient toraise the temperature in the central portion of the cavity to and abovethe vaporization temperature, vapor source material will begin tovaporize from the evaporization regions of wicks 203 and 205. This willcause an increase in pressure in the central portion of chamber 201causing some of this vapor to flow toward window 213 and 215. As thevapor enters the areas enclosed by heat exchanger 209, heat will beremoved from the vapor causing it to condense on the condensationregions of wicks 203 and 205. Arrows 216 indicates the circulatory flowof vapor from the evaporization region to the condensation re gion ofeach wick. As the temperature in the central portion of chamber 201 israised to higher than that needed to produce a vapor pressure equal tothe neutral pressure controlled by pressure control 247, the vapor willpump out all the neutral gas atoms from the central portion of chamber201 into the end portions of chambers 201, the circulatory flow ofvapor. The boundary between the neutral gas and the vapor will besharply-defined, within a few means path lengths, at a position wheresufficient heat has been removed from the vapor so that it can condenseon the condensation region of each wick. The central volume of chamber201 will be filled with pure vapor and the volume adjacent to window 213and 215 will be filled with pure neutral gas which was originallyintroduced into chamber 201.

As the vapor condenses on the condensation regions of wicks 203 and 205,it is pulled by a surface tension forces through the wicks to theevaporation regions of the wicks where it is re-evaporated to create acomplete cycle. Through the use of the principle of the heat pipe asapplied by this invention, a small amount of vapor source material canlast indefinitely.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Examples have been recited, such as discharge chamber of annular shapewith correspondingly shaped wicks and equivalent means for providing thepower or controlling the temperature and/or pressure and exchanging theheat. An example application other than lasers to which this inventionwould be well suited, is shown in the figure, on page 39, of ScientificAmerican Magazine, May, 1968 issue; In this figure, heat is applied tothe center of the heat pipe, which transfers the heat to both ends, in amanner which is similar to heat flow in a vapor laser.

What is claimed is:

1. A discharge cell comprising:

a discharge chamber having an electrically insulating inner surface;

a plurality of wicks, each wick having an evaporization region and acondensation region, said wicks being located within said chamberbetween spaces having elements to be protected from condensate of avapor source material and a central portion of said discharge chamber;

a vapor source material absorbed within said wicks;

a source of electrical power connected between the evaporization regionsof at least two of said wicks for initiating and maintaining anelectrical discharge within said chamber;

a heat exchanger means surrounding those portions of said dischargechamber containing condensation regions of said wicks for maintainingthe temperature within said portionsof said chambers at the condensationtemperature of said vapor source material, such that vapor which isevaporated from each of said evaporation regions circulates to thecorresponding condensation region and returns to the evaporation regionby capillary action within the wick.

2. A discharge cell as in claim 1, wherein:

said elements are a plurality of windows in said discharge chamber, eachwindow being adjacent to one of said wicks, and positioned so that saidwicks are between said windows and a central portion of said dischargechamber.

3. A discharge cell as in claim 1, wherein:

said elements are a pair of mirrors, each mirror being adjacent to oneof said wicks and positioned so that said wicks are between said mirrorsand a central portion of said discharge chamber so that said mirrorsform a resonant cavity within said discharge chamber.

4. 'A discharge cell suitable for use in a vapor laser comprising:

a discharge chamber of electrically insulating material;

a pair of electrodes located within said chamber, said electrodes beingseparated from each other by a discharge path;

a pair of wicks, each wick having an evaporation region and acondensation region, said wicks being located between said electrodeswithin said chamber, said wicks being separated from each other;

a vapor source material absorbed within said wicks;

a source of electrical power connected between said electrodes foriniating and maintaining an electrical discharge within said chamber,said electrical discharge maintaining the temperature at saidevaporation regions of said wicks at the evaporation temperature of saidvapor source material;

a pair of windows, each of said windows being adjacent to one of saidelectrodes, and positioned so that said wicks are between said windows;

a temperature controlling means surrounding portions of said dischargechamber containing condensation regions of said wicks for maintainingthe temperature within said portions of said chamber at the condensationtemperature of said vapor source material, such that vapor which isevaporated from each of said evaporation regions circulates to thecorresponding condensation region and returns to the evaporation regionby capillary action within the wick thereby preventing the vapor fromcondensing on said windows.

5. A discharge cell as in claim 4 wherein said discharge chamber furthercomprises:

a gas inlet orifice connected means for introducing neutral gas andcontrolling the pressure thereof, thereby controlling the vapor densitywithin the discharge chamber.

6. A discharge cell as in claim 5 further comprising:

second temperature controlling means surrounding a central portion ofsaid discharge chamber, said central portion of said discharge chambercontaining evaporation regions of said wicks, for maintaining thetemperature within said central portion of said discharge chamber at theevaporation temperature of said vapor source materi- 7. A discharge cellas in claim 4 further comprising:

second temperature controlling means surrounding a central portion ofsaid discharge chamber, said central portion of said discharge chambercontaining evaporation regions of said wicks, for maintaining thetemperature within said central portion of said discharge chamber at theevaporation temperature of said vapor source material.

1. A discharge cell comprising: a discharge chamber having anelectrically insulating inner surface; a plurality of wicks, each wickhaving an evaporization region and a condensation region, said wicksbeing located within said chamber between spaces having elements to beprotected from condensate of a vapor source material and a centralportion of said discharge chamber; a vapor source material absorbedwithin said wicks; a source of electrical power connected between theevaporization regions of at least two of said wicks for initiating andmaintaining an electrical discharge within said chamber; a heatexchanger means surrounding those portions of said discharge chambercontaining condensation regions of said wicks for maintaining thetemperature within said portions of said chambers at the condensationtemperature of said vapor source material, such that vapor which isevaporated from each of said evaporation regions circulates to thecorresponding condensation region and returns to the evaporation regionby capillary action within the wick.
 2. A discharge cell as in claim 1,wherein: said elements are a plurality of windows in said dischargechamber, each window being adjacent to one of said wicks, and positionedso that said wicks are between said windows and a central portion ofsaid discharge chamber.
 3. A discharge cell as in claim 1, wherein: saidelements are a pair of mirrors, each mirror being adjacent to one ofsaid wicks and positioned so that said wicks are between said mirrorsand a central portion of said discharge chamber so that said mirrorsform a resonant cavity within said discharge chamber.
 4. A dischargecell suitable for use in a vapor laser comprising: a discharge chamberof electrically insulating material; a pair of electrodes located withinsaid chamber, said electrodes being separated from each other by adischarge path; a pair of wicks, each wick having an evaporation regionand a condensation region, said wicks being located between saidelectrodes within said chamber, said wicks being separated from eachother; a vapor source material absorbed within said wicks; a source ofelectrical power connected between said electrodes for iniating andmaintaining an electrical discharge within said chamber, said electricaldischarge maintaining the temperature at said evaporation regions ofsaid wicks at the evaporation temperature of said vapor source material;a pair of windows, each of said windows being adjacent to one of saidelectrodes, and positioned so that said wicks are between said windows;a temperature controlling means surrounding portions of said dischargechamber containing condensation regions of said wicks for maintainingthe temperature within said portions of said chamber at the condensationtemperature of said vapor source material, such that vapor which isevaporated from each of said evaporation regions circulates to thecorresponding condensation region and returns to the evaporation regionby capillary action within the wick thereby preventing the vapor fromcondensing on said windows.
 5. A discharge cell as in claim 4 whereinsaid discharge chamber further comprises: a gas inlet orifice connectedmeans for introducing neutral gas and controlling the pressure thereof,thereby controlling the vapor density within the discharge chamber.
 6. Adischarge cell as in claim 5 further comprising: second temperaturecontrolling means surrounding a central portion of said dischargechamber, said central portion of said discharge chamber containingevaporation regions of said wicks, for maintaining the temperaturewithin said central portion of said discharge chamber at the evaporationtemperature of said vapor source material.
 7. A discharge cell as inclaim 4 further comprising: second temperature controlling meanssurrounding a central portion of said discharge chamber, said centralportion of said discharge chamber containing evaporation regions of saidwicks, for maintaining the temperature within said central portion ofsaid discharge chamber at the evaporation temperature of said vaporsource material.